REQUIRED PRACTICAL - osmosis (potatoes)

• Why is calculating % change in mass better than just calculating change in mass?


  You can compare results that did not have the same initial value.
• Explain why only the water molecules will move across the partially permeable membrane, and not the salt or sugar molecules?

  Water molecules are smaller so will fit through the spaces in the partially permeable membrane whereas sugar or salt molecules are larger and will not be able to fit through.
• Why would this method work with other vegetables, aquatic organisms etc?

  All of these tissues contain water and are surrounded by a partially permeable membrane so they would all undergo osmosis when placed in solution.
• What has to be considered when doing this experiment with aquatic organisms?

  • Don't use too high/low concentrations that could be harmful.
  • Use optimum temperature for them
  • Take care when measuring their mass as to not harm them
• How can you use your graph to estimate the concentration of the solution within the potato cells?

  1. Plot a graph of concentration (x axis) vs % change in mass (y)
  2. Draw a line of best fit
  3. Where the line of best fit crosses the x axis, there is a % change of mass of 0 so this value of concentration is the same as the cell inside.
• What is an application of this experiment in everyday life?

  Having the correct salt concentration in an aquarium so that the fish do not die, depending on the fish-type.
• Ways to modify the method to investigate different independent variables?

  • investigate different types of vegetables
  • investigate wider range of concentrations
  • investigate different solutes
• Possible sources of error in method?

  • potatoes not dried thoroughly, making mass too high
  • potatoes squeezed too much during drying, (mass too low)
  • potatoes not same starting mass/surface area
• Ways of improving the method to reduce sources of error?

  • only dab potatoes lightly to remove surface moisture
  • ensure potatoes have same mass and/or surface area
• Explanation for results?

  • lower salt concentration - higher concentration of water outside potato cells so water moves into cells
  • medium salt concentration - some water concentration inside and outside cells
  • higher salt concentration - higher concentration of water inside potato cells so water moves out of cells
• Expected results?

  • low salt concentration - potatoes gain mass
  • medium salt concentration - little changed mass
  • high salt concentration - potatoes lose mass
• Equipment?

  • 4x boiling tubes
  • 4x potato cylinders
  • 4 different concentrations of salt
  • china graph pencil
• Method?

  1. Label the boiling tubes with the four concentrations of the salt solution.
  2. Record the mass of a potato cylinder and then put it in a boiling tube, write the mass of the potato on the boiling tube.
  3. Add enough of the lowest salt concentration to completely cover the potato.
  4. Repeat steps 2 and 2 with the other three solutions.
• Independent variable?


  concentration of salt solution (%)
• Dependent variable?


  mass of potato (percentage change)
• Control variables and how to keep them constant?

  • the same potato used
  • measure length/width of potato
  • cut skin off of potato
  • same volume of liquid used
  • same weight of potato
  • temperature
• Hazard, risk and control measures?

  • knife, you could cut yourself, only use knife on white tile and carry the knife safely
• Aim?


  Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.